It is known to transmit power over data lines to power remote equipment. Power Over Ethernet (PoE) is an example of one such system. In PoE, limited power is transmitted to Ethernet-connected equipment (e.g., VoIP telephones, WLAN transmitters, security cameras, etc.) from an Ethernet switch. DC power from the switch is transmitted over two sets of twisted pair wires in the standard CAT-5 cabling. The same two sets of twisted pair wires may also transmit differential data signals, since the DC common mode voltage does not affect the data. In this way, the need for providing any external power source for the “Powered Devices” (PDs) can be eliminated. The standards for PoE are set out in IEEE 802.3, incorporated herein by reference.
Providing power over data lines is applicable to other existing systems and future systems. Various new systems using power over data lines may be standardized by the IEEE or other groups.
Although the present inventions may be applied to any system using power over data lines, a typical PoE system will be described as an example.
FIG. 1 represents a typical Ethernet system using PoE. In the example of FIG. 1, a “Power Sourcing Equipment” (PSE) 12 may be any Ethernet device that supplies power and data to a PD. The PSE 12 and PD 14 are typically connected via a standard CAT-5 cable terminated with the standard Ethernet 8-pin (four twisted pairs) connector. Only two of the twisted pairs are typically needed for PoE and data, so there are two spare pairs of wires.
The PSE 12 is typically powered by the mains voltage (120 VAC) and uses either an external or internal voltage converter 16 to generate a DC voltage between 44-57 volts. The PoE standards require the PoE to supply a minimum of 37 volts at the PD. The voltage drop along the cable increases with distance.
Two of the twisted pairs of wires 18 and 20 are assigned to carry the PoE power, and these pairs may also carry differential data. The remaining two pairs or wires 21 and 22 are also shown. All pairs in use are terminated at the PD 14 by transformers, such as transformers 23 and 24. It is assumed that the wires 18 provide 44 volts and the wires 20 are connected to ground. A connection is made to the center tap of transformers 23 and 24 to provide the 44 volts to the PD 14. Since the DC voltage is common mode, it does not affect the differential data. Other conventional termination circuitry is also included in the termination block 25, such as polarity correction circuitry and switches.
The 44 volts is applied to a DC-DC converter 26 for converting the voltage to any voltage or voltages required by the PD 14. The load 28 (e.g., a security camera) is powered by the converter 26 and communicates with the PSE 12 via the twisted wire pairs.
The IEEE standards require certain low current handshaking procedures between the PSE 12 and PD 14 in order to detect the presence of a PoE-powered device and in order to convey the pertinent characteristics of the PSE 12 and PD 14 prior to the PSE 12 making the full power available to the PD 14. The detection/classification circuitry 30 carries out the routine and provides the classification pulses. The PSE 12 also contains circuitry for controlling the handshaking routine.
Below is a simplified summary of the handshaking protocol between the PSE 12 and the PD 14.
When a PoE-enabled Ethernet cable is plugged into the PD 14, the PSE 12 interrogates the PD 14 to determine if it is PoE-enabled. This period is termed the detection phase. During the detection phase, the PSE 12 applies a first current limited voltage for a fixed interval to the PD 14, via the wires 18 and 20, and then applies a second current limited voltage for a fixed interval, while looking for a characteristic impedance of the PD 14 (about 25K ohms) by detecting the resulting current. If the correct impedance is not detected, the PSE 12 assumes that the load is not PoE-enabled and shuts down the PoE generating end. The system then operates as a standard Ethernet connection.
If the signature impedance is detected, the PSE 12 moves on to an optional classification phase. The PSE 12 ramps up the voltage to the PD 14. The PSE 12 generates either one pulse (indicating it is a Type 1 PSE) or two pulses (indicating it is a Type 2 PSE). The PD 14 responds to the classification pulses with certain current levels to identify whether the PD 14 is Type 1 or Type 2. A Type 1 PD requires less than 13 W. A Type 2 PD requires up to a maximum of 25.5 W. Various classes (e.g., five classes), each associated with a maximum average current level and a maximum instantaneous current level, within these types may also be identified. A classification resistance may be used. The PSE 12 then uses this power demand information to determine if it can supply the required power to the PD 14, and the PD 14 uses the information to determine if it can fully operate with the PSE 12. There are maximum time windows for the detection and classification phases (e.g., 500 ms).
Other standards may be implemented.
On completion of the detection and classification phases, the PSE 12 ramps its output voltage above 42 V. Once an under-voltage lockout (UVLO) threshold has been detected at the PD 14, an internal FET is turned on to couple the full voltage to the DC-DC converter 26 to power the load 28. At this point, the PD 14 begins to operate normally, and it continues to operate normally as long as the input voltage remains above a required level.
Recently, it has been proposed to supply up to 51 W (or more) to a PD via the four pairs of wires 18, 20, 21, and 22 by supplying up to 25.5 W using the data wires 18 and 20 and up to 25.5 W using the spare wires 21 and 22, while still complying with the IEEE standards for PoE handshaking
FIG. 2 illustrates a proposed system by Cisco Systems referred to as Universal PoE or UPoE. PSE1 and PSE2 may be conventional Type 2 PSEs and each supplies up to 25.5 W (and up to 30 W in some proposed systems). Each supplies about 44 volts across their associated pairs of wires 44-47 to a single PD 50. The PD 50 uses a conventional 8-pin Ethernet connector. PSE1 and PSE2 may be located in the same Ethernet switch 51 and each may have identical power supplies and detection/classification circuitry. PSE1 and PSE2 may operate independently and do not need to communicate with each other.
A conventional diode bridge polarity correction circuit 52 and 53 for each of the two channels ensures the correct voltage polarity is applied to the load 56, such as 44 volts at the top terminal and zero volts at the bottom terminal
Conventional PD interface controllers 58 and 59, one for each channel, provide the detection resistor 60 (about 25K ohms) and a programmable classification current source 61. At the end of a successful handshaking routine, the controllers 58 and 60 turn on their respective MOSFETs 62 and 64 to supply the 44 volts across the load 56. The load 56 may include a DC-DC converter for converting the 44 volts to any voltage required by the load 56. The body diodes of the MOSFETs 62 and 64 are shown.
In another prior art embodiment, the MOSFETs 62 and 64 are connected in series with the ground conductor, rather than the positive voltage conductor, to create the load current loop for a single channel.
The controllers 58 and 59 and the PSEs (PSE1 and PSE2) perform their detection and classification routines independently and in parallel. Since the PSE1 and PSE2 are assumed to be identical and they share the same Ethernet cable, it is assumed that the final voltages supplied by the PSE1 and PSE2 to the PD 50 inputs are identical (e.g., 44 volts).
An extra set of diodes 66 and 68 is needed to prevent the power from a first channel (e.g., the PSE1 channel) from feeding into the second channel (e.g., the PSE2 channel). This allows the detection and classification parameters of one channel to not be affected by the other channel. The extra diodes 66 and 68 also allow the “negative voltage” bridge diodes to turn on, since, otherwise, the ground voltage from one channel would be at the anodes of the “negative voltage” bridge diodes in the other channel after one of the MOSFETs 62 or 64 turned on, preventing those “negative voltage” diodes from becoming forward biased.
Once both MOSFETs 62 and 64 have been turned on, the power from PSE1 and PSE2 is supplied in parallel to the load 56. This is typically up to 51 W but may be up to 60 W.
Accordingly, in the UPoE system of FIG. 2, there are three diode drops in each channel's power loop, causing a total of about 2.5 W of wasted power at the maximum load power of about 51 W. Other drawbacks exist in the system of FIG. 2.
What is needed is a system for combining the powers from two PSE channels with a higher efficiency than the prior art.